Importing normal maps from obj/mtl
This is obviously an old post that perhaps no one answered. However for the sake of information I will attempt to answer.
Firstly have you atlased the normal map and diffuse map?
This consists of using something like Unwrapper Pro to open the model and move each component part into an individual location to produce one set of UV's for the entire model. Then print out the UV's as a template and cut and paste the relevant parts of the various diffuse and normal maps to all fit onto one image. That is one image for the Diffuse Map and one for the Normal map.
Obj files can have both mesh groups and material groups. So while your model retains all the mesh groups you can select the entire set of UVs and assign it to one material group. So instead of having 100 material groups you have one and this should allow you to only have to set the normal map once.
This is the reverse of what most modelers do where they usually set up only one model group but have many material groups. This is also helpful if you have an animated model as mesh groups are usually where bones are added or weighted.
cheers